Hitherto, there has been known a tensioner used for a timing chain for driving camshafts and others of an engine to apply tension to the timing chain by exerting a force for projecting a plunger slidably and fittingly inserted into a housing and forming an oil chamber between the plunger and the housing by a spring and an external hydraulic pressure.
As such prior art tensioner, a ratchet-type tensioner 500 as shown in FIG. 16 is adopted for example. The ratchet-type tensioner 500 includes a piston 526 slidably and fittingly inserted into a housing 512 in a direction orthogonal to a sliding direction of a plunger 514 and forming a sub-oil chamber 520 between the piston 526 and the housing 512, an oil passage 544 for causing external hydraulic pressure act on the sub-oil chamber 520, an air chamber 528 containing a second spring 534 for biasing the piston 526 toward the sub-oil chamber 520, provided on the side opposite from the sub-oil chamber 520, and sectioned and formed by the housing 512 and the piston 526, an air communicating hole 532 provided in communication with the air chamber 528 and closed by the piston 526 when the external hydraulic pressure acts on the sub-oil chamber 520 and the piston 526 moves against a biasing force of the second spring 534, rack teeth 538 engraved on a part of the plunger 514 surrounded by the housing 512, and a plurality of engaging teeth 536 capable of engaging with the rack teeth 538 and provided at an edge of a rod 524 fixed to the piston 526, wherein tooth surfaces for blocking retraction of the plunger of the engaging teeth 536 and the rack 538 are formed at right angles with respect to a direction in which the plunger 514 advances/retracts (see Patent Literature 1 for example).